He was part of the international collaborative project organized by Mikhail Lomonosov to measure the distance to the Sun, by observing the transit of Venus at different points on the earth. Edmond Halley had suggested the idea, but it required careful measurements from different places on earth, and the project was launched with more than a hundred observers dispatched to different parts of the globe, for observing the transit coming up in 1761. The French expedition turned out to be particularly unlucky, and perhaps the most unfortunate was Guillaume Le Gentil, who set out for Pondicherry, a French possession in India.[1] He set out from Paris in March 1760, and reached Isle de France (now Mauritius) in July. However, war had broken out between France and Britain in the meantime, hindering further passage east. He finally managed to gain passage on a frigate that was bound for India's Coromandel Coast, and he sailed in March 1761 with the intention of observing the transit from Pondicherry. Even though the transit was only a few months away, on 6 June, he was assured that they would make it in time. The ship was blown off-course by unfavorable winds and spent five weeks at sea. By the time it finally got close to Pondicherry, the captain learned that the British had occupied the city, so the frigate was obliged to return to Isle de France. When 6 June came the sky was clear, but the ship was still at sea, and he could not take astronomical observations with the vessel rolling about.[2] After having come this far, he thought he might as well await the next transit of Venus, which would come in another eight years (they are relatively infrequent, occurring in pairs 8 years apart, but each such pair is separated from the previous and next pairs by more than a century.)

After spending some time mapping the eastern coast of Madagascar, he decided to record the 1769 transit from Manila in the Philippines. Encountering hostility from the Spanish authorities there, he headed back to Pondicherry, which had been restored to France by peace treaty in 1763, where he arrived in March 1768. He built a small observatory and waited patiently. At last, the day in question (4 June 1769) arrived, but although the mornings in the preceding month had all been lovely, on this day the sky became overcast, and Le Gentil saw nothing. The misfortune drove him to the brink of insanity, but at last he recovered enough strength to return to France.

The return trip was first delayed by dysentery, and further when his ship was caught in a storm and dropped him off at Île Bourbon (Réunion), where he had to wait until a Spanish ship took him home. He finally arrived in Paris in October 1771, having been away for eleven years, only to find that he had been declared legally dead and been replaced in the Royal Academy of Sciences. His wife had remarried, and all his relatives had "enthusiastically plundered his estate".[3] Lengthy litigation and the intervention of the king were ultimately required before things were normalized. He got back his seat in the academy, remarried, and lived apparently happily for another 21 years.

One of his interesting findings was that the duration of the lunar eclipse of 30 August 1765 was predicted by a Tamil astronomer, based on the computation of the size and extent of the earth-shadow (going back to Aryabhata, 5th century), and was found short by 41 seconds, whereas the charts of Tobias Mayer were long by 68 seconds.[4]